Chemical Bonding
Chemical Bonding
This domain tests why atoms combine, how ionic, covalent, coordinate and metallic bonds form, and how bonding explains melting point, solubility and electrical conduction.
Core rules
- Atoms gain, lose or share electrons to attain a stable noble-gas configuration.
- Ionic bond: transfer of electrons from a metal to a non-metal, forming cations and anions held by electrostatic attraction.
- Covalent bond: sharing of electron pairs between non-metals; one shared pair is a single bond, two a double bond, three a triple bond.
- Coordinate bond: both electrons in the shared pair come from one atom, as in the ammonium ion.
- Metallic bond: positive metal ions in a sea of delocalised electrons.
- Ionic compounds have high melting points, are usually soluble in water and conduct only when molten or in aqueous solution.
- Simple covalent substances have low melting points and generally do not conduct; giant covalent substances such as diamond are hard and high melting.
Worked example
Sodium (2, 8, 1) loses one electron to form Na⁺; chlorine (2, 8, 7) gains one to form Cl⁻. The ions Na⁺ and Cl⁻ attract electrostatically in a 1:1 ratio, giving the formula NaCl. Solid NaCl does not conduct because ions are fixed in the lattice; molten NaCl conducts because ions become mobile. The bond is ionic, not a shared pair.
Traps
Do not call NaCl a molecule in the simple covalent sense; it is a formula unit of an ionic lattice. Covalent compounds can be polar and dissolve in water. Conduction requires mobile charged particles, not just ions present. A coordinate bond is still a covalent-type shared pair.